Varactor-Loaded H-Shaped Antenna With Radiation Pattern Control

A varactor-loaded H-shaped antenna with radiation pattern control is described. The antenna has a varactor in each of the four arms. A cylindrical wire model with four varactors is analyzed by the method of moments. The analysis, using the “FEKO” simulator, shows the figure-of-eight radiation pattern is rotated by 360$^{circ}$ in the plane of the antenna with variation of capacitances of the four varactors. An optimum set of capacitances is derived, considering the beamwidth, null level of the radiation pattern, and voltage standing wave ratio (VSWR). A prototype antenna implemented on FR4 substrate has dimensions of 60 mm by 200 mm with a line width of 2 mm. The dc voltages were supplied to the four varactors via the antenna arms where slits were inserted. A 110 $Omega$ feed line and 110 $Omega$-to-50 $Omega$ balun were designed for the minimum VSWR, whilst maintaining good radiation properties. The antenna exhibits rotation of a figure-of-eight radiation pattern with a VSWR less than 2.7, and the maximum gain from ${-}2.9$ dBi to 1.4 dBi at 750 MHz.